Optical components made of plastic are increasingly used in a wide variety of imaging, telecommunications, medical, defense, and other applications. Because they can be inexpensively molded in high volumes, plastic optical components provide a low cost alternative to conventional glass optics. Plastic lens elements have proved to be particularly advantageous where highly aspheric surfaces or multi-faceted lens elements are needed.
In conventional fabrication of plastic lenses, a mold is designed, machined, and finished to the precise shape requirements of the lens. A suitable high-grade thermoplastic material is then formed into a lens element using the mold in the injection molding process. A wide range of polymers can be used for lens element fabrication in this way, including acrylic (PMMA), polycarbonate (PC), polystyrene (PS), cyclic olefin polymer (COP), and a host of other materials.
As an alternative to molding, some types of plastics can also be machined to obtain optical quality surfaces, with varying degrees of success. Where this capability is available, it allows fast prototyping of optical components, particularly useful for testing and for research and development efforts, without the risk of capital investment in a mold that may not be reusable. However, because molding is the predominant method used for plastic optics fabrication, and because the degree of machining precision needed to obtain optical quality is not possible with many types of polymers, there has been less attention paid to defining and developing useful principles for precision machining of plastics in optical applications.
The high level of precision and accuracy needed for machining an optical surface can be provided by Single-Point Diamond Turning (also termed SPDT) machines, such as those manufactured by Moore Nanotechnology Systems of Keene, N.H. Such diamond turning apparatus are used for shaping optical components of various spherical, aspherical, toroidal, cylindrical, conic, and piano shapes, for example.
Many thermoplastics can be diamond turned to fairly coarse accuracy of within +/−0.1 mm, for example, sufficient for forming components used in a variety of commercial products. Unfortunately, however, only a subset of known thermoplastics can be diamond turned to the high levels of precision needed for forming optical surfaces, where peak-to-valley RMS surface roughness can be no greater than 100 Angstroms and is preferably much less than that amount. Several types of acrylics, for example, have been found to provide acceptable results for optical components using diamond turning methods; however, even these materials can prove difficult to work with except under specific conditions. Many polycarbonate materials prove too soft for diamond-turning to optical standards.
It is widely recognized in the optical fabrication arts that a plastic material must have suitable surface energy characteristics for precision diamond turning. Among other characteristics, a particular material must have suitable durometer, or hardness, in order to be effectively diamond-turned. Some polycarbonates, as noted above, are simply too soft. Other types of plastic prove too brittle for single-point diamond turning.
As a general observation, a number of low-index plastic materials have been found to be workable for precision diamond turning to optical standards. However, as has been acknowledged by at least one plastic optics fabricator, high-index plastic materials have proven to be much more difficult to tool. High index thermoplastic resins have an index of refraction greater than about 1.60.
Among plastics of growing interest for telecommunications and other applications are those having high transmittivity at red and near infrared (IR) light wavelengths, particularly from about 1200 nm to 1600 nm. Two amorphous thermoplastic resins having this transmittivity property are polyetherimide (PEI), manufactured and marketed by General Electric Company, Pittsfield, Mass. as ULTEM® and polyethersulfone (PES), manufactured and marketed by Solvay Advanced Polymers L.L.C., Alpharetta, Ga. as RADEL® A. Having relatively high indices of refraction (about 1.68 for PEI), high dimensional stability, and good resistance to chemicals and fatigue, both PEI and PES are particularly promising candidates for demanding applications using light in the IR region. Their high thermal properties make PEI and PES particularly advantageous for use in optical fiber couplers in data communications and telecommunications applications. Having high glass transition temperatures and being thermally stable at temperatures in excess of 200 degrees C., these plastics can effectively withstand the high levels of heat required for wave reflow solder processing in printed circuit board fabrication.
The use of PEI for lens elements is disclosed in the following, for example:
U.S. Patent Application Publication No. US 2003/0235050 by West et al. discloses a lens component used for a side-emitting LED;
U.S. Pat. No. 6,807,336 to van Haasteren discloses a molded lens formed from ULTEM.
As resins, PEI and PES have been primarily developed and marketed as thermoplastics for injection molding. As noted above, this allows lens elements to be fabricated inexpensively from these materials. According to product literature provided for these resins, precision machining operations, if described at all, are secondary operations at best, that may be employed for specialized use of these materials. As is noted in literature provided by various plastic components fabricators, PEI and PES, without filler materials, are acknowledged to be particularly difficult to machine. The durometer or hardness of stock PEI or PES thermoplastics is very high, not amenable to precision single-point diamond turning. For example, a number of plastic component fabricators, in comparing the overall machinability of various plastics, rate PEI and PES as significantly more difficult to machine than other optical plastics; with a rating of at least 7 on a scale from 1-10, where 10 indicates the most difficult.
To make versions of these thermoplastics that are more suitable for machining operations, manufacturers mix them with various glass fillers and other filler materials. However, while these filler materials allow easier machining, they render such thermoplastics as unusable for optical applications. Thus, optical-grade PEI and PES materials, being difficult to machine except to coarse precision, appear to offer little promise as candidates for high-precision optical machining.
For prototyping, as well as for small production runs, it would be highly advantageous to be able to fabricate lens elements from these and similar high index, high thermal property thermoplastic materials using single-point diamond turning. However, these materials, as supplied by the manufacturers, are particularly poorly suited to single-point diamond turning at the precision needed for optical quality. Conventional procedures for handling and pre-shaping lens blanks from these materials do not yield components having a compatible surface for diamond turning to form an optical surface. Therefore, it is widely held among those skilled in the optical plastic fabrication arts that PEI and PES, and similar types of high index, high thermal property thermoplastics, cannot be satisfactorily diamond turned to the optical quality needed for prototype or production-quality lens elements.
Conventional mold design is particularly costly, with typical mold prices often ranging in the tens of thousands of dollars. This makes molding a particularly expensive way to develop prototype plastic lens elements for initial development and testing efforts. Moreover, mold fabrication can take considerable time, often requiring 12 weeks or more. Time-to-market considerations can be highly significant, particularly in telecommunications, where short product lifetimes are anticipated for many types of components. Diamond turning, while not a preferred method for mass manufacture, potentially offers, to those who need prototypes, reduced lead times in terms of months and cost savings in terms of tens of thousands of dollars.
Thus, it can be seen that there would be particular benefits to methods and apparatus that provide single-point diamond-turned lens elements of high optical quality from PEI, PES, and similar high index, high thermal property thermoplastics.